The present invention generally relates to a thermal head which is used in a thermal transfer recording apparatus, a heat sensitive recording apparatus, etc. for printers, facsimiles, etc.
Conventionally, the thermal transfer recording apparatus and the heating sensitive recording apparatus for printers, facsimiles, etc. effect the heat sensitive recording with respect to a heat sensitive paper or an ordinary paper with an ink sheet superposed thereon by the use of a thermal head. The thermal head to be used in printing apparatuses such as a thermal transfer apparatus, thermally sensitive printing types of printers, etc. is two in types as follows. A first one is a so-called thin membrane type, wherein heating resistors, electrodes for energization use and abrasion-proof layers are formed by a vacuum thin membrane forming process such as evaporation, sputtering on a glaze-alumina base plate so as to form patterns by the use of a photolitho etching method. A second one is named so-called thick membrane type, wherein electrodes for energization use, heating resistors, and abrasion-proof layers are respectively formed on a glaze-insulation base plate by the printing burning of the paste.
The above-described two types of thermal heads have advantages and disadvantages respectively. Namely, as the thin membrane type of thermal head is uniform in its resistor shape (area, thickness, etc.) among the respective dots, with its thermal capacity being uniform, the heat transfer into the paper is uniformly effected during the printing operation. Also, as the resistance values of the respective resistors are obtained uniformly up to some extent, and the thermal head is collectively superior in the print quality. As the thickness of the resistor is as thin as 1000 to 5000 .ANG., the thermal capacity is smaller, with the constant becoming superior, the print heating efficiency becoming higher during the rising and falling operations of the resistor temperature during the on and off pulse application. However, in the conventional thin membrane type, it is difficult to have the dispersion of the resistance value at .+-.5% or lower, so that a more superior print quality is hard to obtain. Also, there are many problems to be solved in terms of productivity, lower cost such as facility cost, batch production, etc. for the thin membrane process.
On the other hand, it is noteworthy that the thick membrane type of thermal head has many advantages such as lower facility cost and easier continuous production, because it uses a print burning method.
FIG. 5 is a construction view of the conventional thick membrane type of thermal head. A glaze layer 2 is formed on the top face of an alumina base plate 1. A common electrode 3, an individual electrode 4 and a heating resistor 5 are formed on it, with an abrasion layer 6 being provided to cover the respective one portion of the heating resistor 5, the electrodes 3, 4.
FIG. 6 is a plan view showing the electrode shape of the conventional thick membrane type of thermal head. As it is difficult to independently constitute the heating resistor in the thick membrane type of thermal head, a line-shaped common heating resistor 5 is provided, with the conductive electrodes for energization use 3 and 4 having the common electrode 3 and the individual electrode 4 introduced and exposed in a zigzag shape, alternately from both the sides of the heating resistor 5. Also, one dot is constructed in one individual electrode 4, with two heating portions 7a and 7b being provided correspondingly. Namely, upon the application of voltage in pulse upon between one individual electrode 4 and a common electrode 3,, a current flows at the same time to the heating portions 7a and 7b to form two color forming points.
Conventionally the resistor values of the heating member of the thick membrane type thermal head having the electrode shape of the zigzag type have the dispersion of ten-odd percent in a plurality of dots within the sam head. The major causes for the resistance value dispersion lay in nonuniformity in the dispersion condition, etc. of the heating resistor material, and printing accuracy in uniformity, etc. of the line width, thickness of the line-shaped common heating resistor 5. Namely, in the thick membrane type of thermal head, it is difficult to uniformly print the line width of the line-shaped common heating resistor 5 enough to have several percent of dispersion, so that the contact area between the electrodes 3 and 4 for energization use introduced, disposed from both the sides of the heating resistor 5 and the heating resistor 5 is different, thus resulting in fundamentally increasing the dispersion of the respective dot resistance values.
Therefore, the resistance value of the dot may be uniformly adjusted into approximately .+-.1% through a trimming operation by the use of an energization overload trimming system (a method of using the resistance value variation through self-generating Joule heat to be caused when the power is fed into the heating resistor), but the calorific value per unit value of the heating resistor can not be made uniform.